1. Role of High Added Power Efficiency Amplifiers in Mobile Telecommunications Equipment
The rapid growth of the mobile telecommunications industry has created an increased demand for low-cost, low-power and lightweight equipment. Since mobile phones are essentially battery-operated devices, the amount of time that a mobile phone can be used before requiring recharging will be largely dependent on the power consumption of its transceiver circuitry.
In general, the power consumption of the oscillator and filter components of a transceiver circuit is negligible compared with that of the circuit's RF power amplifier. Consequently, the growing demands of the mobile telecommunications industry have focussed attention on the design and in particular the power efficiency of RF power amplifiers.
The efficiency of a power amplifier can be increased by minimising its power dissipation whilst ensuring that the output power is maintained at the desired level. However, it should be noted that the instantaneous power consumed by an active device is defined as the product of the drain-to-source voltage and drain-to-source current at any given point in time. Switching power amplifiers increase their efficiency by ensuring that a non-zero voltage does not exist across the amplifier at the same time that a non-zero current is flowing through the amplifier. In other words, switching power amplifiers ensure that either the voltage across the amplifier and/or the current flowing through the amplifier is zero at any given point in a RF cycle. Consequently, the product of the voltage and the current is zero at all times and in an ideal case no power is consumed by the device (i.e. all the DC input power to the power amplifier is converted to RF power and the DC-to-RF frequency of the amplifier is 100%). This functionality is essentially achieved through a switching operation that is inherently non-linear in character. The class E power amplifier is a well-known switching power amplifier.
2. Traditional Class E Power Amplifiers
(a) Circuit Structure
FIG. 1 is an equivalent circuit diagram for a conventional class E power amplifier. The circuit comprises an active device (e.g. a bipolar junction transistor (BJT), heterojunction bipolar transistor (HBT), junction field-effect transistor (JFET), metal-oxide-silicon field-effect transistor (MOSFET) or metal semiconductor field effect transistor (MESFET)), a RF choke 1 and surrounding passive elements that form a load network.
Unlike conventional linear amplifiers (where the output of the active device varies smoothly in proportion with the input signal), the active device in a class E power amplifier operates as a switch having two discrete states, namely ON and OFF. Bearing this in mind, the active device in FIG. 1 is represented by a switch 2. The opening and closing of the switch 2 is controlled by a RF input signal (not shown).
During the ON state the switch 2 is closed and acts as a low resistance element which permits current to flow through it. During the OFF state the switch 2 is open and thus acts as a high impedance element capable of withstanding a voltage rise across its terminals. The operation of the switch 2 stimulates oscillation in a moderate-Q series resonant circuit 3 (comprising an inductor Ls and a capacitor Cs) which is tuned to the fundamental frequency of the switching waveform. The oscillation of the resonant circuit 3 forces a sinusoidal current through a load resistor RL, which results in a sinusoidal output signal iθ. The flow of a constant current throughout an entire RF cycle is ensured by applying a sufficiently large DC bias signal (vDC) to the active device through the RF choke 1. The RF choke 1 typically has a very large inductance and thus only allows a DC current to pass through it.
A shunt capacitor Cp is connected in parallel with the active device (switch 2). The task of the shunt capacitor Cp is to act as a temporary energy store while the switch 2 is open thereby allowing the resonant circuit 3 to carry on oscillating even when the switch 2 is closed (i.e. all the current is flowing through the switch 2).
(b) Modes of Operation
(i) OFF State Operation
During the OFF state (switch 2 open), the DC current vDC and the output current iθ flows through the shunt capacitor Cp (i.e. forming current ic). This current charges the shunt capacitor Cp, resulting in a non-zero voltage vc (not shown). During the negative half-swing of the output current iθ, the current into the shunt capacitor (ic) becomes negative and discharges the shunt capacitor Cp. If the switch timing and the capacitance of the shunt capacitor Cp are chosen correctly the shunt capacitor Cp is fully discharged (i.e. there is no voltage across the output terminals of the active device) by the time the switch 2 opens.
(ii) ON State Operation
During the ON state, the voltage across the switch 2 is zero (in the ideal case) and the current flows entirely through the switch 2 (i.e. thereby forming isw).
Consequently, at no time during the operation of the amplifier is a current flowing through the switch 2 and a voltage dropped across the switch 2. Consequently, the amplifier circuit consumes no energy and is ideally 100% DC-to-RF efficient.
In a real class E amplifier the switch timing is set via the gate bias and the voltage over the shunt capacitor Cp can be determined through the following equation.
            i      c        ⁡          (      t      )        =            ⅆ                        v          c                ⁡                  (          t          )                            ⅆ      t      (c) Problems with Conventional Class E Power Amplifiers
In spite of its obvious advantages, the mobile telecommunications industry has so far been slow to adopt the class E power amplifier. This stems from the inherent sensitivity of the amplifier to parasitic resistances existing within the active device, especially drain-to-source channel resistance. Class E power amplifiers have mainly been used to date to amplify lower frequency VHF signals where the parasitic resistance of the active device is not as problematic.
Class E amplifiers are inherently sensitive to these parasitic resistances because the currents flowing through the active device are relatively high. Consequently, even small parasitic resistances cause a significant voltage drop across the active device that interferes with the operation of the circuit during switching. This effect is particularly pronounced when operating at high frequencies.
As a consequence, with few exceptions all microwave designs to date have used heterojunction bipolar transistors (HBTs) as active devices, since their collector resistances are smaller (by about a factor of 10) than the drain-source resistance of a typical pseudomorphic high electron mobility transistor (pHEMT) with 100 μm gate width (R. E. Anholt, Electrical and thermal characterization of MESFETs, HEMTs, and HBTs. Norwood, Mass., USA: Artech House, Inc., 1995). In addition, the on conductance of a pHEMT drain varies with the gate width, further complicating the design process of the class E power amplifier.
The few pHEMT-based class E amplifier designs published to date use very large devices with total gate widths in the range 1 mm (IEEE Radio Frequency Integrated Circuits (RFIC) Symposium Digest, Pages 53 to 56, Seattle Wash., USA, June 2002) to 12 mm (European Conference on Wireless Technology, Pages 181 to 184, 2001). These devices operate by parallelling multiple transistor drain resistances to reduce their overall effect on circuit behaviour. However, the beneficial effect of this approach is achieved with the expense of additional device manifold losses.
3. Amplification of Modulated Signals
(a) Phase Shift Keying (PSK)
Phase-shift keying (PSK) is a method of transmitting and receiving digital signals in which the phase of a transmitted signal is varied to convey information. There are several schemes that can be used to accomplish PSK including biphase modulation (BPSK) and Quadrature Phase Shift Keying (QPSK). QPSK is widely used in code division multiple access (CDMA) cellular services, wireless local loop and digital video broadcasting-satellite (DVB-S).
Since the informational content of a QPSK signal is contained within its phase, the necessity for linear amplification is not as extreme as with QAM signals. However, since large envelope variations can occur during phase transitions, linear amplification of QPSK signals is nonetheless generally required. Consequently, a class E power amplifier would not normally be considered for the amplification of QPSK signals.
(b) Quadrature Amplitude Modulation (QAM)
QAM is a modulation scheme that combines amplitude modulation and phase shift keying to transmit several data bits per symbol and is primarily used in microwave digital radio, digital video broadcasting-cable (DVB-C) and modems. For example, eight state quadrature amplitude modulation (i.e. 8QAM) signals typically have four phase states and two amplitude states (i.e. two concentric rings on a constellation plot, each ring corresponding with a particular amplitude state and comprising four points corresponding with the phase states).
The informational content of a QAM signal is contained in the amplitude of the signal. Consequently, if it is necessary to amplify a QAM signal, the amplification should be performed with a linear amplifier in order to preserve the informational content of the signal. Similarly, it should not be theoretically possible to preserve the informational content of a QAM signal if it is subjected to non-linear amplification. For example, if a non-linear amplifier is used to amplify an 8QAM signal, the resulting signal should in theory possess four states because the outer concentric ring from the constellation plot of the original signal would have collapsed onto the inner concentric ring. In view of the above, a class E power amplifier would not normally be considered for the amplification of QAM signals.
Linear power amplifiers are typically less efficient than switching power amplifiers since current flows through the active device at the same time as there is a voltage across the device for at least a portion of a RF cycle. Since both QAM and QPSK signals generally require linear amplification, the transceiver circuits used for transmitting and receiving such signals are characterised by high power consumption.